This invention relates to supports for lithographic printing plates, particularly to supports that can be processed into lithographic printing plates having longer press life. More specifically, the invention relates to (1) supports that can be processed into lithographic printing plates having longer press life, higher stain resistance and better surface quality, (2) supports that can be obtained by the process comprising efficient electrochemical graining treatment and which can be processed into lithographic printing plates that have longer press life and which retain this property even after the plate surface is wiped with a plate cleaning solution, and (3) supports that can be processed into lithographic printing plates having longer press life and higher resistance to aggressive ink staining.
Photosensitive lithographic printing plates using aluminum alloy plates as supports are extensively used in offset printing. Such lithographic printing plates are prepared by processing presensitized plates. Generally, the presensitized plate is made by roughening the surface of an aluminum alloy plate, anodizing it, applying a photosensitive solution, and drying the applied coat to form a photosensitive layer. The presensitized plate is exposed imagewise, whereupon the exposed areas of the photosensitive layer change in physical properties. The photosensitive layer is then treated with a developer solution so that it is removed from the exposed areas (if the presensitized plate is positive-acting) or from the unexposed areas (if the presensitized plate is negative-acting). The areas from which the photosensitive layer has been removed are hydrophilic nonimage areas and the areas where the photosensitive layer remains intact are ink-receptive image areas. Thus, presensitized plates are processed into lithographic printing plates using the changes in the physical properties of the photosensitive layer that take place upon exposure.
The lithographic printing plate is then mounted on the plate cylinder for printing. In printing, an ink and a fountain solution are supplied to the surface of the plate. The ink adheres only to the image areas of the plate and the image is transferred to the blanket cylinder, from which it is transferred to the substrate such as paper, thereby completing the printing process.
Aluminum alloy plates are conventionally grained by three known techniques, mechanical (e.g. ball graining and brush graining), electrochemical (electrolytic etching with a liquid electrolyte based on hydrochloric acid, nitric acid, etc.; this technique is also hereunder referred to as xe2x80x9celectrolytic grainingxe2x80x9d), and chemical (etching with an acid or alkali solution). Since the plate surfaces prepared by electrolytic graining have homogeneous pits and exhibit better printing performance, it is common today to combine the electrolytic graining method with another method such as mechanical graining or chemical graining.
By electrolytic graining, aluminum alloy plates acquire roughened surfaces that have not only xe2x80x9cwavyxe2x80x9d or xe2x80x9cwrinkledxe2x80x9d asperities to an average surface roughness Ra of about 0.30-1.0 xcexcm but also pits in honeycomb or crater form that are about 0.2-20 xcexcm in diameter and about 0.05-1 xcexcm in depth. If the xe2x80x9cwavyxe2x80x9d or xe2x80x9cwrinkledxe2x80x9d asperities in the roughened surface obtained by the electrolytic graining method are not adequately uniform, it is preferably combined with the mechanical and/or chemical graining method to increase the uniformity of the asperities.
If the pits formed in the plate surface by graining are not uniform in diameter or depth, several defects occur and this problem is hereunder described with reference to FIGS. 1 and 2 which show schematically the cross-sectional structure of a conventional presensitized plate indicated by 10. As shown, the presensitized plate 10 consists of an aluminum alloy support plate 12 having pits P formed in its surface which in turn is coated with a photosensitive layer 14. First suppose that the pits P do not have uniform depth in the direction of exposure (see FIG. 1A); if the area where a deeper pit Pxe2x80x2 is formed is exposed, halation (nonuniform scattering of light) occurs (see FIG. 1B) and not only the exposed areas but also the unexposed areas change in physical properties (see FIG. 1C). This may produce xe2x80x9cfogxe2x80x9d in the printed image. If the presensitized plate is exposed over a wide area including pits P and the deeper pit Pxe2x80x2 (see FIG. 2A), the exposure at the bottom of the pit Pxe2x80x2 which is far from the light source (see FIG. 2B) may turn out insufficient to produce a yet-to-be exposed portion in the xe2x80x9cexposed areasxe2x80x9d (see FIG. 2C). The areas from which the photosensitive layer have been removed should inherently become nonimage areas but on account of such yet-to-be exposed portion, the nonimage areas will partly show the characteristics of the image areas. This portion is most likely to become the start point for staining to occur during printing.
Another problem with the nonuniformity of the asperities in the roughened plate surface is decreased adhesion between the photosensitive layer and the support, which in turn leads to a shorter press life of the lithographic printing plate. While direct imaging presensitized plates (for laser platemaking) are drawing increasing attention these days, longer press life is more desired since the adhesiveness of the photosensitive layer to the support is more susceptible than the photosensitive layer of the conventional presensitized plate which requires photographic films during platemaking. Uniformity of the asperities in the plate surface is extremely important to laser platemaking since insufficient exposure is all the more likely to occur.
Therefore, when roughening the surface of an aluminum alloy plate, pits that have appropriate depth and diameter and which are uniform in size must be generated uniformly in the entire surface of the support so that the photosensitive layer adheres strongly to the support while allowing the aluminum alloy plate to hold more water. The deeper the pits, the stronger the adhesion between the photosensitive layer and the support.
As mentioned above, the nonuniformity of the roughened surfaces of supports for lithographic printing plates have considerable effects on press life and other parameters to the printing performance of lithographic printing plates. In order to deal with this problem, many proposals have been made that try to eliminate the nonuniformity by changing the aluminum alloy composition of the plates. Many proposals have also been made concerning the waveform and frequency of the power supply for electrolytic graining.
In offset printing, ink is not directly transferred from the plate onto the substrate such as paper; instead, as shown in FIG. 3, ink 4 on the lithographic printing plate 1 wrapped around a plate cylinder 5 is first transferred to an elastic rubber coat (blanket 2) wrapped around a transfer cylinder 6 and the blanket 2 carrying the layer of ink 4 and the substrate 3 supplied by an impression cylinder 7 are brought into contact under sufficient pressure to perform printing.
If the pits in the nonimage areas are not uniform, the fountain solution is only insufficiently held in the nonimage areas to prevent the ingress of ink which, therefore, adheres to the nonimage areas of the plate surface to stain it. The stain is transferred to the blanket and eventually appears as stain on the print. In order to prevent this problem of stained prints, the pressman who has noted a stain on the blanket usually stops the press, cleans off the ink from the nonimage areas and supplies an increased amount of the fountain solution to prevent further staining of the plate surface. Cleaning is done by wiping the entire plate surface including both image and nonimage areas with a sponge imbibed with a suitable amount of an acidic or alkali liquid plate cleaner. This removes the ink adhering to the nonimage areas of the plate surface.
However, cleaning the entire plate surface with the liquid plate cleaner has its own problems. The applied liquid cleaner either swells the photosensitive layer to lower its strength or permeates between the photosensitive layer and the support to reduce their adhesion. If the cleaned plate is used to print many copies, wear or separation of the photosensitive layer is likely to occur in the solid image areas that are extensively rubbed with the blanket or in the highlight areas which adhere only slightly to the support. Therefore, lithographic printing plates are required to retain long press life even after their surface is cleaned with a liquid plate cleaner (this characteristic is also hereunder referred to as xe2x80x9cpress life after cleaner applicationxe2x80x9d).
As already mentioned, the adhesion between photosensitive layer and support is an important factor to providing lithographic printing plates with longer press life both before and after cleaner application. This adhesion is greatly influenced by pit depth, diameter, their uniformity, as well as the uniformity in distribution of the pits in the support surface and the density of their distribution and many RandD efforts have been made to improve these factors. An additional recent requirement is for lower cost in graining treatments; to meet this need, it is desired to generate the intended pits within a shorter period of time by raising the efficiency of electrolytic etching in the electrolytic graining treatment.
With a view to producing uniform roughened surfaces on supports for lithographic printing plates, it has been proposed that uniform graining by electrolytic etching be ensured by incorporating 0.05-0.1 wt % of Cu in an aluminum alloy support containing 0.05-1 wt % of Fe and 0.01-0.15 wt % of Si (JP-A-11-99763).
According to another proposal, it is described that the Fe, Si and Cu levels in an aluminum alloy support are adjusted to the ranges of 0.05-1 wt %, 0.015-0.2 wt % and xe2x89xa60.001 wt %, respectively, with the distributed elemental Si level in the metal structure being regulated to 0.015 wt % or more and the uniformity in surface roughening by electrolytic etching, fatigue strength and burning characteristics are improved (JP-A-11-99764).
According to yet another proposal, it is described that the Fe, Si and Cu levels in an aluminum alloy support are adjusted to the ranges of 0.05-1 wt %, 0.015-0.2 wt % and 0.001-0.05 wt %, respectively, with the distributed elemental Si level in the metal structure being regulated to 0.015 wt % or more and no streaks occur and uniformity in surface roughening by electrolytic etching, fatigue strength and better burning characteristics are improved (JP-A-11-99765). This method produces uniform pits by a short period of electrolytic graining treatment.
According to a further proposal, it is described that the Fe, Si and Ti levels in an aluminum alloy support are adjusted to 0.20-0.6 wt %, 0.03-0.15 wt % and 0.005-0.05 wt %, respectively, with part or all of these elements forming intermetallic compounds and the number of the grains of said intermetallic compounds present on the surface and of a size between 1 and 10 xcexcm being regulated to 1000-8000 grains/mm2 and pits can be formed by a short period of electrolytic graining treatment without producing unetched areas and uniform pits can be formed by roughening treatment even if they are shallow (JP-A-11-115333).
It has also been proposed that roughening pits be formed uniformly by adjusting the Fe, Si, Ti and Ni levels in an aluminum alloy support to 0.20-0.6 wt %, 0.03-0.15 wt %, 0.005-0.05 wt % and 0.005-0.20 wt %, respectively, with part or all of these elements forming intermetallic compounds which are regulated to contain Al, as well as Fe, Si and Ni in respective amounts of 20-30 wt %, 0.3-0.8 wt % and 0.3-10 wt % (JP-A-9-279272).
It has also been proposed that roughening pits be formed uniformly by adjusting the Fe, Si, Ti, Ni, Ga and V levels in an aluminum alloy support to 0.20-0.6 wt %, 0.03-0.15 wt %, 0.005-0.05 wt %, 0.005-0.20 wt %, 0.005-0.05 wt % and 0.005-0.020 wt %, respectively, with the Ti, Ga and V contents being regulated to satisfy the relation 1xe2x89xa6([Ti]+[Ga])/[V]xe2x89xa615, where [Ti], [Ga] and [V] represent the contents (wt %) of Ti, Ga and V, respectively (JP-A-9-279274).
According to yet another proposal, it is described that the Fe, Si and Cu levels in an aluminum alloy support are adjusted to the ranges of 0.05-1 wt %, 0.01-0.2 wt % and xe2x89xa60.031 wt %, with either Ni or Cr or both being contained in an amount of 0.003-0.1 wt % and uniformity in surface roughening by electrolytic etching are improved (JP-A-11-D99760).
Also proposed is an aluminum alloy plate that contains Fe, Si, Ti and Ni in respective amounts of 0.20-0.6 wt %, 0.03-0.15 wt %, 0.005-0.05 wt % and 0.005-0.20 wt %, with either Cu or Zn or both being contained in an amount of 0.005-0.05 wt % and at least one element of the group consisting of In, Sn and Fb being contained in an amount of 0.001-0.020 wt % (JP-A-9-272937). Using this aluminum alloy plate, one can generate uniform pits by a short duration of electrolytic graining treatment.
However, if the Cu content of aluminum alloy supports is zero or very small (xe2x89xa60.001 wt %) as proposed in JP-A-11-115333, JP-A-11-99764, JP-A-9-279272, JP-A-9-279274 and JP-A-11-99760, supra, no deep enough pits are generated and the supports have short press life and low stain resistance. Also problematic is the micro-streaking (unevenness in the form of very fine streaks) that results from low Cu levels.
The aluminum alloy support proposed in JP-A-11-99765, supra has such a large content (xe2x89xa70.015 wt %) of elemental Si (which is one of the four forms in which Si occurs in aluminum alloy supports) that defects will readily develop in the anodized coat, leading to frequent occurrence of aggressive ink staining. The term xe2x80x9caggressive ink stainingxe2x80x9d will be explained later in detail and suffice it here to say that when printing is done with the occurrence of many interruptions, the nonimage areas of the lithographic printing plate have so much increased ink receptivity on the surface that stain appears as spots or rings in the print (e.g. paper) and this stain is referred to as xe2x80x9caggressive ink stainingxe2x80x9d.
Conversely, if aluminum alloy supports contain Cu in large amounts (xe2x89xa70.05 wt %) as proposed in JP-A-11-99763, there is no problem of xe2x80x9cmicro-streakingxe2x80x9d which occurs in the case of low Cu content but, on the other hand, no uniform electrolytic graining can be achieved and xe2x80x9cyet-to-be etchedxe2x80x9d, or undergrained, areas are prone to occur, leading particularly to poor stain resistance.
The supports having such undergrained areas suffer from the disadvantage of deteriorated surface quality since they have fine glossy areas on the surface.
According to JP-A-9-272937, the support that has no xe2x80x9cyet-to-be-etchedxe2x80x9d areas caused by insufficiency of electrolytic graining and has highly uniform grained surface observed by SEM can be obtained by a short duration of electrolytic graining treatment. However, the test about printing performance was not done. As it was put to the test actually, printing performance, particularly press life that requires the adhesion between photosensitive layer and support, more particularly press life after cleaner application was insufficient.
The Assignee previously proposed that an aluminum alloy support containing 0.05-0.5 wt % of Fe, 0.03-0.15 wt % of Si, 0.006-0.03 wt % of Cu and 0.010-0.040 wt % of Ti, with at least one of 33 elements including Li, Na, K and Rb being contained in an amount of 1-100 ppm and with the purity of Al being regulated to 99.0 wt % or higher, should be subjected to graining treatments including electrolytic graining so as to produce a support for lithographic printing plates that has been grained with high efficiency to give a very high degree of uniformity in the grained surface (JP2000-37965A).
The thus produced support for lithographic printing plates had high uniformity in pits, or highly uniform grained surface, and lithographic printing plates prepared from this support had longer press life and other improvements in printing performance. However, even this support could not necessarily be processed into a lithographic printing plate having satisfactory resistance to aggressive ink staining.
This is not a prior art, but the Assignee filed Japanese Patent Application 11-349888 and taught that when the aluminum alloy support disclosed in JP2000-37965A, namely, the one containing specified amounts of Fe, Si, Cut Ti and at least one of 33 elements including Li, Na, K and Rb, was modified by further incorporating a specified amount of Mg, its surface could be uniformly roughened by electrochemical graining to provide a support for lithographic printing plates that was suitable for platemaking using a laser light source.
Even this support could not necessarily be processed into a lithographic printing plate having satisfactory resistance to aggressive ink staining.
Therefore, a first object of the invention is to provide an aluminum alloy support for lithographic printing plates which can be processed into lithographic printing plates having longer press life, higher resistance to staining and better surface quality.
A second object of the invention is to provide an aluminum alloy support for lithographic printing plates that can be obtained by the process comprising efficient surface roughening by electrolytic graining and which can be processed into lithographic printing plates having longer press life both before and after application of a liquid plate cleaner.
A third object of the invention is to provide an aluminum alloy support for lithographic printing plates which can be processed into lithographic printing plates having longer press life and higher resistance to aggressive ink staining.
The present inventors started with the conventional aluminum alloy support containing Fe, Si, Cu and Ti as essential ingredients, They additionally incorporated Mg as an essential ingredient and further incorporated a specified amount of Ni as yet another essential ingredient. Then, in accordance with the value of the average surface roughness Ra of the support surface that was to be achieved by graining, the contents of Cu and Ni were adjusted to lie within specified ranges. As a result, large and deep graining pits could be generated uniformly to leave no residual fine glossy areas on the support which hence could be processed into lithographic printing plates having longer press life, higher resistance to staining and better surface quality. Thus, the first object of the invention could be attained.
According to its first aspect, the present invention provides a support for a lithographic printing plate which is obtained by performing surface roughening treatment including electrochemical graining on an aluminum alloy plate containing 0.2-0.5 wt % of Fe, 0.04-0.20 wt % of Si, 0.005-0.040 wt % of Cu, 0.010-0.040 wt % of Ti, 0.001-0.020 wt % of Mg and 0.005-0.2 wt % of Ni, with the balance being Al and an incidental impurity, and which satisfies the following relation (1):
[Ni]/10+[Ra]/100xe2x89xa6[Cu]xe2x80x83xe2x80x83(1) 
where [Ni] and [Cu] are the Mi and Cu contents (wt %), respectively, of said aluminum alloy plate, and [Ra] is the average surface roughness Ra (xcexcm) of the roughened support surface.
The present inventors also started with the conventional aluminum alloy support containing Fe, Si, Cu and Ti as essential ingredients. They additionally incorporated Mg as an essential ingredient and further incorporated 0.001-0.05 wt % in total of at least one element selected from among In, Pb, Sn, Bi, Cr, Mn and Zn. As a result, a support for lithographic printing plates in which the depth and diameter of pits generated by electrolytic graining (hereunder sometimes referred to as xe2x80x9celectrolytic graining pitsxe2x80x9d), the uniformity of these size parameters, the uniformity in distribution of these pits in the support surface and the density of their distribution were adjusted to lie within the desired ranges could be produced with a small quantity of electricity; the support could be processed into lithographic printing plates having longer press life before and after cleaning with a liquid plate cleaner. Thus, the second object of the invention could be attained.
According to its second aspect, the present invention provides a support for a lithographic printing plate which is obtained by performing surface roughening treatment including electrochemical graining on an aluminum alloy plate containing 0.2-0.5 wt % of Fe, 0.04-0.20 wt % of Si, 0.005-0.040 wt % of Cu, 0.010-0.040 wt % of Ti, 0.001-0.020 wt % of Mg and 0.001-0.05 wt % in total of at least one element selected from among In, Pb, Sn, Bi, Cr, Mn and Zn, with the balance being Al and an incidental impurity.
The present inventors conducted extensive studies on the improvement of resistance to aggressive ink staining with a view to attaining the third object of the invention. As already mentioned, presensitized plates are a dual-layered structure consisting of an aluminum alloy support plate having pits formed in its surface and which is overlaid with a photosensitive layer, After imagewise exposure of the plate surface, development is performed to make nonimage areas from which the photosensitive layer has been removed and image areas where the photosensitive layer remains intact to record an image on the surface. For printing, ink and a fountain solution are supplied to the image-bearing lithographic plate, so that the fountain solution adheres to the nonimage areas and the ink adheres to the image areas, from which it is transferred to the substrate such as paper via a blanket.
If the printing operation is interrupted, the residual fountain solution in the nonimage areas evaporates slowly and the solutes become concentrated, sometimes attacking the anodized coat on the surface of the nonimage areas. As many interruptions occur, the surface of the nonimage areas having the anodized coat attacked by the solutes in the fountain solution loses hydrophilicity to become ink receptive and the problem of xe2x80x9caggressive ink stainingxe2x80x9d (the development of stains in the form of spots or rings on the substrate such as paper) occurs.
The Assignee found that the chlorine ions in the fountain solution took significant part in the phenomenon of aggressive ink staining and proposed a method of screening them out (JP-A-11-301241).
Making further studies on the problem of aggressive ink staining which would take place on aluminum alloy supports for lithographic printing plates that incorporated Mg as an essential ingredient in addition to Fe, Si, Cu and Ti contained as essential ingredients in the conventional aluminum alloy support, the present inventors noted that nonuniformity in pit depth was one of the causes of aggressive ink staining and found that uniformity in pit depth could principally be controlled by the Si content of the aluminum alloy plate and that only when it maintained a specified relationship with the Cu content could the Si content be an effective factor in controlling the uniformity of pit depth,
The present inventors further found that if the nonimage areas had large water holding capacity (could hold more of the fountain solution on their surface), their surface was prone to be attacked during the above-described process of evaporation of the fountain solution; it was also found that the average surface roughness Ra was an effective characteristic value for expressing the water holding capacity of the nonimage areas. To be specific, the smaller the average surface roughness Ra, the smaller the water holding capacity and the less likely was the aggressive ink staining to occur. On the other hand, water holding capacity is a very important factor to the printing performance in other aspects and the present inventors found that an unduly small water holding capacity was not preferred and there was an advantageous range favoring a good balance between the various aspects of the printing performance. On the basis of these findings, the present inventors accomplished a support for lithographic printing plates that could attain the third object of the invention.
It was also found that if the Si content of the aluminum alloy plate was excessive, there was a high likelihood for Si itself to be the point at which the attack of the nonimage areas would start as the fountain solution evaporated, so there was a recognized need to control the Si content to lie within an appropriate range in accordance with the average surface roughness Ra. This finding also led to the accomplishment of the support for lithographic printing plates that could attain the third object of the invention.
In short, the present inventors found the following two facts about the aluminum alloy support for lithographic printing plates that incorporated Mg as an essential ingredient in addition to Fe, Si, Cu and Ti contained as essential ingredients in the conventional aluminum alloy support: the press life of the lithographic printing plate into which the support was processed and its resistance to aggressive ink staining could be controlled by adjusting the Cu and Si levels; the resistance of the plate to aggressive ink staining was influenced by the average surface roughness Ra. On the basis of these findings, the present inventors successfully obtained a support for lithographic printing plates that was free from the defects of the conventional lithographic support, which suffered only limited defects in the anodized coat and which, hence, could be processed into lithographic printing plates having markedly improved printability, longer press life and higher resistance to aggressive ink staining.
Thus, according to its third aspect, the present invention provides a support for a lithographic printing plate which is obtained by performing surface roughening treatment including electrochemical graining and anodization on an aluminum alloy plate containing 0.2-0.5 wt % of Fe, 0.04-0.20 wt % of Si, 0.005-0.040 wt % of Cu, 0.010-0.040 wt % of Ti and 0.002-0.020 wt % of Mg, with the balance being Al and an incidental impurity, and which has an average surface roughness Ra of 0.2-2.0 xcexcm after said roughening treatments and which satisfies the following relation (2):
xe2x88x922[Cu]+0.14xe2x88x92[Ra]/10xe2x89xa6[Si]xe2x89xa6xe2x88x92[Cu]+0.22xe2x88x92[Ra]/10 xe2x80x83xe2x80x83(2) 
where [Si] and [Cu] are the Si and Cu contents (wt %), respectively, of said aluminum alloy plate, and [Ra] is the average surface roughness Ra (xcexcm) of the roughened support surface.
In the support for lithographic printing plates according to the first, second or third aspects of the invention, said roughening treatments preferably include mechanical graining and/or chemical graining in addition to electrochemical graining.